Actually it is normal. It's a hum from the charger not being grounded. The effect also happens on the MacBooks. You will feel some electrical charge when touching the MacBook. Apple no longer includes the extension cable that grounds the charger. Sadly it's an additional $19 but that's what gets rid of the buzzing and hum.
Not normal on my 16" or my partners 2017 without the extension. Being in the UK could have something to do with it as I think it has ground (three prong)... Dunno, not a sparky.